1. Field of the Invention
This invention relates to an improvement of thread forming fasteners being easily driven into workpieces during the swaging operation and being steadily secured to them with high stripping torque when they are seated, preferably for thin boards and the like.
2. Prior Art
Generally selftapping fasteners or thread forming fasteners are advantageous for being driven into pilot holes drilled and untapped in thin boards and the like. Especially the thread forming fasteners form female threads in the side wall of the pilot hole in the boards by swaging operation. They are more advantageous for being difficult to loose by themselves because of tight engagement between the screw threads and the female threads when they are driven into the holes in the workpieces. And besides the female threads are advantageous for improving thier strength because of work hardening created in themselves. The thread forming fasteners, however have disadvantages for being driven through the workpieces with relatively high torque requirement. To prevent the disadvantages as above discribed, new screws have been developed, which was my Japanese Utility Model application No. 57-36820. The new screw has a driving head at one end with any driving means with which a driver bit may have engagement, and a shank portion with a straight thread formation at the other side. The shank portion is of non-circular or trilobular which may be termed an arcuate equilateral triangular shape in cross section of the blank. And the configuration is characterized by three equally spaced lobes, which are separated by relatively broad arcuate sides. The shank portion consists of the first portion which has a tapered end portion with a screw thread increasing gradually and progressly radial dimensions of the lobes and radial dimension of the broad arcuate sides between two adjacent lobes from the screw axis, the second portion which merges smoothly and continuously with the first portion and has a constant radial dimension from the screw axis to the lobes and has such lobes that have gradually increasing radii and such sides that have gradually decreasing radii, the third portion which merges smoothly and continuously with the second portion and is of a substantially circular shape in its cross section, and a screw thread formed over the first portion, the second portion and third portion.
When a screw is driven into a pilot hole in a workpiece, the first portion can curle a portion around the hole of the workpiece and a female thread is gradually swaged in the curled portion by the first tapered thread portion while the screw thread gradually increases an area of the sides in one revolution of the screw thread engaged with the female thread formed thereby. At this time it will be observed that the total engagement is decreased to amount to some of the total peripheral extent of one turn of the screw thread in the first portion because of the equilateral triangle of the first portion. Therefore the screw can be driven with very low driving torque. The second portion is driven into the pilot hole following the first portion and each lobe of the screw thread over the second portion gradually and progressly swages and enlarges the female thread formed by the first portion in order to complete a swaging operation. At this stage the engagement of the sides with the female thread is gradually increased and therefore a driving torque is increased little by little without introducing surged driving torque, because the configuration of the thread portion is gradually formed and converted from a triangular shape to a round shape. At the last stage the third portion of a substantially round shape in its cross section is driven there following the second portion, wherein the screw thread over it is in tight engagement with the female thread in the curled portion of the workpiece. And besides the cross sectional shape of a substantial circular in the third portion enables the total engagement of the screw thread with the female thread to amount to all the peripheral extent of one turn of the screw thread and to provide high stripping torque of the mating threads as well as the high fail torque after fastening operation.
Although the above discribed screw, has the tapered portion of the first portion for easy entry to a pilot hole punched or pierced in a workpiece, the screw thread over the tapered portion causes the pilot hole much larger in diameter than that of the end of the tapered portion. Therefore, the difference between the root radius of the screw thread over the second portion and the radius of the pilot hole decreases so as to shorten the curled portion along the hole in the workpiece. This causes the screw to have less holding strength. And also there will be introduced disadvantages that some parts of the screw thread over the tapered portion are cracked and the screw is difficult to begin a swage operation because the screw thread enlarges a pilot hole and forms a curled portion around the hole in the workpiece.